Background Field
Embodiments of the subject matter described herein are related generally to optical metrology, and more particularly to target design and manufacture for optical critical dimension metrology.
Relevant Background
During the production of semiconductor devices, such as integrated circuits, it is desirable to measure the circuit structures. Optical metrology tools are particularly well suited for measuring microelectronic structures because they are nondestructive, accurate, repeatable, fast, and inexpensive. The critical dimension of structures, such as gratings, trenches, and contact and holes is often of interest. Optical critical dimension (OCD) metrology, for example, is often used with three-dimensional semiconductor devices.
Typically, OCD metrology utilizes an OCD target that is fabricated at the same time as the structure under test and includes the same features to be characterized. The OCD target, for example, may be produced in a scribe line between semiconductor chips or in otherwise available areas of the chip. Conventional methods of OCD metrology include, e.g., reflectometry, scatterometry, and ellipsometry. These metrology methods can often accurately measure features on an upper surface of a device, but in many cases, the presence of an absorbing layer reduces or eliminates the measurement signal strength from the structures that are below the absorbing layer. Additionally, conventional methods of OCD metrology are often unsatisfactory in accurately determining the bottom CD profile of high aspect ratio structures, both in one dimensional gratings (e.g. deep narrow trenches) and in two dimensional arrays (e.g. contact and via holes). This is particularly true as the CD decreases far below the wavelength of the incident light.